A corona shield is used in many electrical applications, in particular in generators, as described in EP 1 995 850 B1.
In order to avoid partial discharges, the main insulation of generator winding bars at operating voltages of a few kilovolts has to be shielded from cavities and detachments by an inner and an outer conducting layer. The electric field strength is reduced in the main insulation proceeding from the inner potential grading 10 (FIG. 1) (IPG) in the radial direction as far as the outer corona shield 13 (FIG. 1) (OCS). At the end of the generator winding bar, in the region of the exit point of the winding bars from the stator laminate stack, the OCS ends, while the main insulation is continued in the direction of the bar end. This arrangement constitutes a typical sliding arrangement having an extremely low partial discharge inception voltage. In this region the electric field also has, in addition to the radial component, a strong nonlinear tangential component parallel to the insulating material surface/interface. The highest field strength occurs at the end/edge of the OCS. Therefore, it is necessary to provide for a field control at the edge of the outer corona shield and for an increase in dielectric strength in the vicinity of the exposed main insulation. This is usually achieved by the production of an overhang corona shield 16 (FIG. 1). In order to suppress creeping discharges, use is usually made of resistive potential gradings by means of semiconducting varnishes or tapes predominantly on the basis of silicon carbide or other electrically semiconducting fillers.
The aim of the potential grading is to make more uniform, and ideally to linearize, the tangential potential reduction along the insulating material surface. This is achieved if the absolute value of the voltage drop per unit length is always the same. A resistance per unit length that is voltage-dependent and location-dependent in the axial direction is produced for this purpose.
In this case, the time duration for sufficiently curing and solidifying hitherto commercial materials is very long particularly in the case of varnishes, but also in the case of tapes, since across a plurality of work shifts a plurality of layers of varnish have to be applied and between applications it is necessary to wait for a certain time interval in order that the subsequent layer can be applied again by overcoating.
The overhang corona shield is realized nowadays either by single- or multi-ply wrapping with electrically semiconducting tapes or by applying one or a plurality of layers of an electrically semiconducting varnish.
The semiconducting tapes usually consist of an electrically nonconductive carrier material (e.g. polyester nonwoven, polyester fabric or glass fabric) and a reaction resin (e.g. epoxidized phenol novolaks, often accelerated by means of dicyandiamine) in a prereacted stage (B-stage). For complete curing, tapes of this type have to be cured for 2 hours at approximately 165° C. or for up to 12 hours at only 120° C. Silicon carbide is usually used as filler nowadays, wherein the average grain size determines the resulting electrical resistance of the tape.
Semiconducting varnishes are typically solvent-based systems such as phenolic resins comprising semiconducting or semiconducting-functionalized fillers.
At room temperature, a time of a plurality of hours (up to 4 or more) is required here to obtain overcoatability. Since it is often necessary to produce up to five layers one above another, this is a time-consuming process.
The high drying time follows from the required high solvent content (approximately up to 30%). This must be rejected, however, on account of environmental protection and occupational safety aspects. In this case there is also the risk of initiating a hidden defect. Present-day systems are additionally restricted to low heat stability classes.
Ready-mixed varnishes are offered commercially only for specific resistance ranges. Further resistance ranges are required, however, which are produced manually by dedicated mixing. However, these mixtures have specific disadvantages, such as subsidence/segregation of the filler, the risk of an incorrect mixture, poorer processing properties (e.g. coatability).
Hitherto there have been virtually no satisfactory approaches for accelerating the processing by means of significant reductions of the curing times. The curing times are the slowest step in this manufacturing stage and are thus a speed-determining factor for manufacture.